$ B = \left[\begin{array}{rr}1 & 0 \\ 0 & -2 \\ 5 & -1\end{array}\right]$ $ E = \left[\begin{array}{rr}1 & 4 \\ -2 & -1\end{array}\right]$ What is $ B E$ ?
Answer: Because $ B$ has dimensions $(3\times2)$ and $ E$ has dimensions $(2\times2)$ , the answer matrix will have dimensions $(3\times2)$ $ B E = \left[\begin{array}{rr}{1} & {0} \\ {0} & {-2} \\ \color{gray}{5} & \color{gray}{-1}\end{array}\right] \left[\begin{array}{rr}{1} & \color{#DF0030}{4} \\ {-2} & \color{#DF0030}{-1}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ E$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ E$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ E$ , and so on. Add the products together. $ \left[\begin{array}{rr}{1}\cdot{1}+{0}\cdot{-2} & ? \\ ? & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ E$ and add the products together. $ \left[\begin{array}{rr}{1}\cdot{1}+{0}\cdot{-2} & ? \\ {0}\cdot{1}+{-2}\cdot{-2} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ B$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ E$ and add the products together. $ \left[\begin{array}{rr}{1}\cdot{1}+{0}\cdot{-2} & {1}\cdot\color{#DF0030}{4}+{0}\cdot\color{#DF0030}{-1} \\ {0}\cdot{1}+{-2}\cdot{-2} & ? \\ ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{1}\cdot{1}+{0}\cdot{-2} & {1}\cdot\color{#DF0030}{4}+{0}\cdot\color{#DF0030}{-1} \\ {0}\cdot{1}+{-2}\cdot{-2} & {0}\cdot\color{#DF0030}{4}+{-2}\cdot\color{#DF0030}{-1} \\ \color{gray}{5}\cdot{1}+\color{gray}{-1}\cdot{-2} & \color{gray}{5}\cdot\color{#DF0030}{4}+\color{gray}{-1}\cdot\color{#DF0030}{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}1 & 4 \\ 4 & 2 \\ 7 & 21\end{array}\right] $